(a) Field of the Invention
The present invention relates to a method for calculating a loading amount of ammonia in a selective catalytic reduction apparatus. More particularly, the present invention relates to a method for calculating a loading amount of ammonia in a selective catalytic reduction apparatus, which maximizes the reduction rate of nitrogen oxide and prevents slip of ammonia as a consequence of precisely calculating the amount of ammonia loaded in the selective catalytic reduction apparatus.
(b) Description of the Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the exhaust gas undergoes a decrease in noise while passing through a muffler and is emitted into the air through a tail pipe. The catalytic converter is a type of a diesel particulate filter (DPF) and purifies pollutants contained in the exhaust gas. A catalytic carrier for trapping particulate material (PM) contained in the exhaust gas is in the catalytic converter, and the exhaust gas flowing out from the engine is purified through chemical reaction.
One type of catalytic converter is a selective catalytic reduction (SCR) apparatus. In the selective catalytic reduction (SCR) apparatus, reductants such as carbon monoxide and total hydrocarbon (THC) react better with nitrogen oxide than oxygen. Thus, such a catalytic converter is called a selective catalytic reduction apparatus. An ammonia-SCR apparatus among the selective catalytic reduction apparatuses uses ammonia as a reductant in order to purify nitrogen oxide. According to the ammonia-SCR apparatus, nitrogen oxide is reduced well and accelerates a reaction between a monoxide and ammonia in a case that oxygen exists. Thus, the ammonia-SCR apparatus has been applicable to a diesel exhaust apparatus.
Such an ammonia-SCR apparatus applied to a vehicle for selectively removing nitrogen oxide is a urea-SCR apparatus. According to the urea-SCR apparatus, a urea solution is dosed upstream of an SCR catalyst and ammonia generated by decomposing urea is used in order to reduce nitrogen oxide. Therefore, the urea-SCR apparatus is a type of ammonia-SCR.
As the dosing amount of urea increases, the reduction rate of nitrogen oxide also increases according to the urea-SCR. However, if the dosing amount of urea is larger than a predetermined amount, a part of dosed urea is not decomposed or remaining ammonia that does not react with nitrogen oxide increases. Therefore, it is important to minimize the dosing amount of urea and to increase the reduction rate of nitrogen oxide according to the urea-SCR apparatus.
In addition, for increasing the reduction rate of nitrogen oxide and preventing slip of ammonia in the SCR apparatus, the actual loading amount of ammonia is controlled to be the same with a target loading amount of ammonia that can be loaded in the SCR apparatus. However, the loading amount of ammonia that is loaded in the SCR apparatus cannot be detected by a sensor and should be estimated.
According to prior arts, the loading amount of ammonia in the SCR apparatus is estimated by subtracting the ammonia amount that reacts with nitrogen oxide from the dosed ammonia amount. In this case, it is assumed that current loading amount of ammonia is not reduced.
However, the loading amount of ammonia in the SCR apparatus changes according to temperature. That is, if the temperature in the SCR apparatus increases, the loading amount of ammonia in the SCR apparatus decreases.
Therefore, the estimated loading amount of ammonia is different from the actual loading amount of ammonia, and accordingly, the reduction rate of nitrogen oxide may be deteriorated or a slip of ammonia may occur according to a conventional method for estimating the loading amount of ammonia in the SCR apparatus.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.